Thursday, December 25, 2014

In today's post, we will study a particular class of firearm that was very uniquely American and popular from the end of the Civil War to the beginning of World War I. We are going to study about Pocket Rifles, otherwise called Bicycle Rifles.

The origin on these weapons has to do with the Stevens Arms and Tools Company, founded by Joshua Stevens. He was a well-respected toolmaker, who had worked for Colt, Eli Whitney, Smith & Wesson, Allen and many other American gunmakers of the era, before founding his own firearms company in 1864. The company's first two models were a Pocket Pistol and a Vest Pocket Pistol (a year ahead of Remington's Vest Pocket Pistol model). In 1869, the company produced what it called a "Pocket Rifle". This was largely based on their Pocket Pistol model, except that it had a longer barrel, better sights and a cap on the pistol's grip to accept a detachable shoulder stock made of wire. Like the Pocket Pistol, the Pocket Rifle was also a single shot model.

In 1872, a larger 'New Model Pocket Rifle' was added to handle cartridges up to .32 caliber rimfire cartridge. Shortly after that, a line called the 'Hunters Pet Pocket Rifle' was also introduced that went up to .44 caliber. The shoulder stock was also modified so that it slid into a dovetail cut into the butt of the pistol and a screw on the backstrap.

Public domain image of Stevens Pocket Rifles

Click on the image to enlarge. Public domain image.

The New Model Pocket Rifle (First Issue) was the same basic design as the Old Model Pocket Rifle, but was larger and had a heavier barrel to handle the bigger .32 caliber rimfire cartridge. It became far more popular than the old model and outsold it by a wide margin. It was only manufactured for three years though, between 1872-1875, after which it was replaced by the New Model Pocket Rifle (Second Issue) model, which was sold from 1875-1896

The second issue model mounted the firing pin in the frame rather than the hammer, as a safety feature. In 1887, a version that fired the .22 Long Rifle (also known as .22 LR) rimfire cartridge was manufactured for the first time. The .22 LR cartridge was also invented by the Stevens Arms and Tools Company and is still the most popular cartridge in the world today (almost every major firearm manufacturer in the world has made at least one product that fires .22 LR).

When separated into two pieces (the pocket rifle and the stock), each piece measured between 18 to 24 inches (46-61cm.), which meant they could be stowed in a long coat pocket. Weight of the larger caliber models was around 5 to 5.75 lbs. (2.2 - 2.6 kg.) and the lighter models up to .32 caliber only weighed about 2 - 2.75 lbs. (0.9-1.25 kg.) The barrels were offered in a variety of lengths: 10 inches, 12 inches, 15 inches or 18 inches (25 cm., 30 cm., 38 cm. or 46 cm.)

In the 1880s, advertisements for these guns started to refer to them as "Bicycle Rifles", probably as a marketing tactic to sell them to cyclists of that era, as a light rifle that could be carried for self defense.

An advertisement for a Stevens Bicycle Rifle. Click on the image to enlarge.

They were also offered with carrying cases made of leather or canvas and marketed to hunters as a secondary light rifle, and to fishermen to carry with their fishing equipment.

The nice thing about these compact rifles was that they offered much more range and accuracy than pistols, but were much cheaper than other single shot rifles of that era, while also being much more portable than other rifle models. One Mr. A.C. Gould reported that using a model firing .22 caliber cartridges with an 18 inch barrel, ten shots were placed into a target of 8 inches diameter at 200 yards distance.

After the success of the initial models, other manufacturers also started to make pocket rifles, but Stevens continued to dominate the market until the last model was manufactured during World War I. It must be noted that practically all dealer catalogs of that period that advertised pocket rifles. invariably showed the Stevens brand name. Some larger dealers offered pocket rifles under their own brand, but many of these were actually manufactured by Stevens and marked with the dealer's brand name.

While pocket rifles sold very well in America, they remained a very American invention and never really spread to other countries. While they were light and relatively portable, they were all single-shot models. Their popularity began to decline after semi-automatic and fully-automatic weapons became more common.

Tuesday, December 16, 2014

So what is a barrel shroud? It is simply a hollow covering tube that surrounds a barrel (either partially or fully). What does it do? Well, it protects the user of the firearm from accidentally burning himself or herself with the hot barrel.

A typical barrel shroud

A barrel shroud typically has many holes throughout its length. The holes serve to reduce its weight and also dissipate heat by venting out hot air. The next picture shows a barrel shroud attached to a firearm.

As you can see in the above image, the barrel shroud is simply that tube with holes that surrounds the barrel. In the above example, the user has also attached an extra hand grip to the barrel shroud. Since much of the barrel shroud is not in contact with the hot barrel, if the user was to accidentally touch the front of the firearm, the user will not get burned by the barrel.

The curious reader is probably thinking now, "isn't that what the stock of a firearm is designed to do?", Yes, the stock and the receiver do protect the user's hands as well, but they are not considered as barrel shrouds, because they serve other purposes as well, whereas the barrel shroud is a separate component that is screwed on around the barrel and explicitly designed to protect the user's hands (or other body parts) from heat.

Barrel shrouds are generally commonly seen with air-cooled machine guns, but they are also optional components for many semi-automatic models. Some shotguns also feature barrel shrouds. There are many third party component makers that make barrel shrouds for various rifle and shotgun models. In general, they are useful to have with weapons that fire rapidly, because the barrel can heat up quite a bit after a few shots in rapid succession.

If a barrel shroud is simply a covering tube to protect a user from touching a hotter part of the gun, then what's the big deal about them? Well, for a while, barrel shrouds were targets of legislative restrictions in the United States. The now expired Federal Assault Weapons Ban explicitly included barrel shrouds in its list of features for which a semi-automatic firearm could be banned (if a firearm had two features in the list, it could be banned under this law). After the law expired, proposals were made to renew the ban, including this provision, but have not been successful so far.

Amusingly, during an interview on MSNBC in 2007, Representative Carolyn McCarthy was asked about her gun control legislation and why it prohibited people from purchasing firearms that have barrel shrouds and if she even knew what a barrel shroud was. After attempting to avoid the questions twice, she finally admitted, "I don't know what it is, I think it is a shoulder thing that goes up!"

It is amazing that she was trying to introduce a law to ban something without even knowing what it was!

Where we last left off, the barrel blanks were straightened out and tested for straightness. The next process was to heat treat the barrel blanks and increase their hardness. We will discuss this heat treating process shortly.

The last process was to grind the ends of each blank and then grind a spot on the enlarged end of each blank and test the hardness of the blank on a Brinell machine, to ensure that the blanks met the required hardness nunbers

The Brinell hardness test was invented by Swedish engineer Johan Brinell in 1900. It was one of the first standardized hardness tests used in engineering and is still used today. The test is very simple. It uses a steel or tungsten carbide ball of diameter 10 mm. (0.39 inches), which is used as an indenter. The ball is placed on the surface of the object to be tested and a 3000 kg. (or 6600 lbf.) test force is applied to the ball for a specific time (normally 10 to 15 seconds). After this, the ball is removed and it leaves a round indentation on the surface of the object. The diameter of this indentation is measured and the Brinell Hardness Number (BHN) is calculated, based upon the diameter of the ball, diameter of the indentation and the force applied to the ball. For softer materials, such as aluminum, a smaller test force (e.g. 500 kg. (or 1100 lbf.) is used instead.

The above image shows a line drawing of the concept and the formula actually used to calculate the Brinell Hardness Number (HB in the image above).

Returning back to our study of the factory process, the barrel blanks were tested for hardness to make sure that they had a Brinell Hardness Number (BHN) of at least 240.

At this point, the barrel blanks were shipped off to a barrel manufacturer, who would then drill, ream, finish-turn and rifle the blanks into complete barrels.

Now. all through the description of the process so far, we've been talking about heating the blanks for various purposes. We will cover the heat treatments in detail here. There were actually four separate heat treatments done to the blanks.

Heating and soaking the steel above the critical temperature and quenching it in oil, to harden the steel through to the center of the blanks.

Reheating the steel for drawing of temper for the purpose of meeting the physical specifications of the blank

Reheating the blanks to meet the machineability test for production purposes

Reheating to straighten out the blanks when hot.

We will study each of the four heating processes in detail.

For the first heating process, the blanks were slowly brought up to the required heat, which is about 150 degrees Fahrenheit (65.5 degrees centigrade) above the critical temperature of the steel. The blanks were then soaked at a high heat for about one hour before quenching in oil. The purpose of this treatment was to eliminate any strains already existing in the bars that may have been put there from milling operations done to the bars. Remember that steel is an elastic substance and working it puts stress on the bars. For instance, during the production of steel, the manufacturer rolls the bars through various rollers to make them the required diameter, which causes the bars to come out stressed. The heat treatment process removed the stress caused by rolling, hammering, cutting etc. It also ensured that the heat treatment applied to the entire cross-section of the bar and not just the surface. In addition, if a blank had seams or slight flaws, these opened up drastically during the quenching process and made it easy to determine if a blank was defective or not.

The oil used for quenching was kept at a temperature of around 100 degrees fahrenheit (38 degrees centigrade). This is an ideal temperature is to prevent shock to the steel when it is dropped into the quenching oil, otherwise it could develop surface cracks on the piece.

The second heating process (the one for drawing the temper of the steel) was a very critical operation and had to be done carefully. The steel had to be kept heated within 10 degrees of temperature fluctuation in the process. The degree of heat necessary for this operation depended entirely on analyzing the steel. Even if the steel was purchased from the same manufacturer, there was always some variation in different batches received from the manufacturer.

The third heating process (reheating for machineability) was done at a temperature of around 100 degrees Fahrenheit (38 degrees centigrade) less than the drawing temperature used for the second heating process. However, the time of soaking was almost double that of the second process.

For both the second and third heating process, after the heating was done. the blanks were buried in lime so that they would be out of contact with air, until their temperature had dropped down to room temperature.

The fourth heating process was used when straightening the blanks. In this process, the blanks were first heated to about 900-1000 degrees Fahrenheit (482-538 degrees centigrade) in an automatic furnace for 2 hours before straightening them. The purpose of heating before the straightening was to prevent any stresses being put into the blanks during the straightening operation. This is necessary because when later processes such as drilling, turning and rifling are done to the blanks, they have a tendency to spring back into the shape they were in when they left the quenching bath. By heating before straightening, the blanks are prevented from doing this.

Another method was later found to produce an even better barrel blank. The blanks were first rough-turned to the final barrel diameter and then heated to about 1000 degrees Fahrenheit (538 degrees centigrade) for about 4 hours before sending them to the barrel manufacturer. Blanks produced with this method remained practically straight during the different barrel making operations (drilling, reaming, finish-turning and rifling). This meant that the barrel manufacturers didn't need to straighten barrels after they were finished (which was a much more expensive operation). This method was tested out with one of the largest barrel manufacturers in the US and it proved to be very effective.

As the reader might be wondering, all this heat-treating needed a large amount of oil for cooling and one of the problems was how to keep all this oil at the proper temperature. After much study, a cooling system was developed for the factory. The next two images show the cooling system as seen on the roof from the outside of the factory.

Click on the images to enlarge. Public domain images.

The next image shows the details of the cooling system:

The hot oil is pumped up from the quenching tanks through the pipe A into the tank B, From here, the oil runs down onto the separators C, which break the oil up into fine particles, that are blown upwards by the fans D. The spray of oil particles is blown up into the cooling tower E, which contains banks of cooling pipes and baffles F. Cold water is pumped through the inside of the pipes. The spray of oil particles collects on the outside of the cold pipes and forms larger drops, which fall downwards onto the curved plates G and then run back to the oil-storage tank below ground. The water pumped through the cooling pipes comes from 10 natural artesian wells at a rate of 60 gallons per minute and this serves to cool about 90 gallons of oil per minute, lowering it from a temperature of about 130-140 degrees Fahrenheit to about 100 degrees Fahrenheit. The water comes out of the wells at an average temperature of 52 degrees Fahrenheit. The pump is driven by a 7.5 HP motor and the speed can be varied to suit the amount of oil to be cooled. The plant was designed to handle up to 300 gallons of oil per minute.

The finished blanks from this factory were sent to different barrel manufacturers to drill, ream, rifle etc. to their requirements.

Tuesday, December 9, 2014

After all the stuff we studied about metallurgy in the last several posts, we will look at an ancillary subject today, forging of rifle barrel blanks. We have already covered barrel manufacture from barrel blanks in some detail in previous posts many months ago. In today's post, we will study the process of manufacturing the barrel blanks as it was done in a factory in America in the 1920s. In particular, this was a factory belonging to Wheelock, Lovejoy & Company, which was designed to mass-produce rifle barrels designed to meet specifications demanded by some foreign governments. Some of the pictures and information in this post was taken from the book "The Working of Steel" by Fred Colvin and K.A. Juthe, and in addition, K.A. Juthe was the designer of the factory as well.

This factory did not manufacture its own steel: instead, they bought what they needed from a large steel manufacturer. The steel manufacturer made the steel to the required specifications and supplied them in the form of bar stock, but the length of the supplied bars was longer, typically each bar measured something like 30 or 35 feet long (about 9.1 to 10.7 meters long).

Cutting the bar stock to size.

Therefore, the first step in the process was to cut each steel bar into smaller lengths to make barrels. The bars came on trucks and were fed through the cutting-off shear, where they were cut into pieces of the proper length. The pieces were actually a little longer than the final barrel lengths, to allow for trimming during the machining process.

A close up of the details of the cutting off is shown in the next image.

A is the stock stop bolted to the side of the frame and the ledge formed by the strip bolted to the stop, keeps the bar stock level during the cutting process. The hold-down B prevents the back end of the steel bar from flying up when the bar is cut. The knife C has several notched edges with which the barrels can be cut, so that it need not be taken out for resharpening, until all the notches are dull.

The cut barrel pieces then passed into the next room, where there was a forging or upsetting press.

Upsetting (or more properly, upset forging) is a process of increasing the diameter of the end of a work piece, by compressing it along its length inside a die. The images below show the process.

The barrel pieces were heated in a furnace to soften them, before being sent through the upsetting press. The press could handle the barrels from all the heating furnaces shown in the room. The men changed work at frequent intervals, to avoid excessive fatigue.

The barrels were then sent through a continuous heating furnace to be reheated and then straightened out as much as possible before being tested for straightness.

A Continuous Heating Furnace

In the above machine, each barrel was tested for straightness by placing it on the rollers as shown in the image above. The screw on the press was used to apply pressure and straighten out the barrel as needed.

Thursday, December 4, 2014

After that long series about different metals used in firearms manufacturing, might as well take a break from a dry topic and watch something else instead.

Initially, your humble editor thought that someone was parodying the infamous Rex Kwon Do scene from the movie Napoleon Dynamite. (In case you haven't seen the movie, here's clip 1 and clip 2). Unfortunately, what you're about to see is not a joke, there is actually someone trying to train people to use tactics like this. This group is called the "Sulsa Do Corps" (no joke, that's what they call themselves). See for yourself:

This video was originally posted at a youtube channel called God Rock Ministries / Expert Karate, which appears to be some sort of combination of church and karate school (dojo). This school appears to be run by a Mr. David Bateman. They removed the original video from the channel, when word about its unintentional comedy started to spread. Unfortunately for them, the video was saved by someone else and here it is :).

It is a darn good thing they are using pellet guns with no ammunition, instead of real pistols. Questionable tactics? Where do we start. First, we have quite a few instances of them sweeping each other with the muzzles of their pistols (big safety no-no). Then, we have fingers placed on the trigger at all times (bad idea). Next we have a couple of cases of shooting at the ceiling and open door, while moving and rolling around (another safety no-no). What's with the backwards dive to the ground anyway and since when is running backwards without looking at where you're going ever a good idea. Then, we have firing the pistols close to own face/someone else's face (if those were real pistols, guess whose eardrums are getting blasted to hell, not to mention hot cartridge brass getting ejected on someone else). We also have thumbs placed behind the slide in some instances (if that was a real pistol, someone is going to have a broken thumb when the slide moves backwards at high speed after the cartridge is fired). Then, they run in front of each other, with the ones in the back shooting (good chance of getting shot in the back if they were using real pistols). Then, there's the firing between the legs position (guess whose kneecap is going to receive some hot cartridge brass if those were real pistols). Firearm sights are there for a reason, but they don't seem to know how to use them. We even have a few instances of triggers pulled when one of the others was in the line of fire. The funniest part in your humble editor's opinion is around 0:13 of the video, when the gentleman starts his solo run backwards, holds his pistol practically next to his cheek, then shoots one through the open door, then falls over backwards and bangs his head against the wall! There's probably a few more bad things I missed because I was laughing too hard.

The scary part is that they appear to be dead serious and actually imagine that this is good training. If those were real pistols, someone is definitely going to get hurt or worse. In case you're wondering, this David Bateman has a few more videos about his martial arts training academy, including this gem:

Tuesday, December 2, 2014

In our last post, we looked at a modern method of manufacturing steel, Basic Oxygen Steelmaking (a.k.a) the BOS process. As we saw, this is based on the Bessemer process, except that we use oxygen instead of air to burn off impurities. When we studied the Bessemer process, shortly after that, we studied how fluid compressed steel was made from steel made by the Bessemer process. The purpose of compressing the steel was to eliminate gas bubbles and hairline cracks in the ingot. Well, the BOS process also could have these problems for the same reasons as well, so we will study how these problems are tackled in today's post.

The problem is that when steel is manufactured using the BOS process, oxygen is injected over the molten metal to burn off impurities. As it turns out, not all of this oxygen gets used up to burn impurities, some of the excess oxygen gets dissolved in the molten steel as well. When the metal solidifies, this oxygen is released out and can do bad things to the steel. For one, it can combine with the iron in the steel, to form iron oxide (i.e. rust). The second is that the oxygen gas can form gas bubbles (blowholes) in the ingot. Thirdly, it can combine with the carbon in the steel, forming carbon monoxide and carbon dioxide, which reduces the carbon content of the steel and weakens it. Also, the carbon monoxide and carbon dioxide gas can form blowholes in the steel as well. Gas bubbles and blowholes cause the steel to have pores in it. One more problem is that the carbon monoxide tends to form more on the outside of the ingot and escapes out. This causes non-uniform distribution of the carbon in the steel, because the outside of the ingot now becomes relatively pure iron, while the inside of the ingot is carbon steel. Also, steel shrinks considerably as it cools and trapped gas in the metal can cause gaps and hairline cracks in the ingot as well. For firearm applications, the presence of rust, bubbles, cracks and pores is undesirable, as is the non-uniform distribution of carbon in the steel.

So clearly, we must minimize the oxygen in the molten steel before it solidifies and preferably remove it without forming a gas like carbon monoxide, because the gas could cause bubbles and cracks to form. In modern times, this is done right after the molten steel is tapped out of the BOS furnace and poured into molds, by adding deoxydizing agents to the molten steel. Basically, a deoxydizing agent is a chemical that strongly combines with oxygen better than carbon and iron do. Therefore, as the molten steel cools, the dissolved oxygen combines with the deoxydizing agents first, before it has a chance to react with the iron or carbon in the steel. A good deoxydizing agent also forms solid slag rather than a gas, so that there are no gas bubbles or cracks formed as the steel cools. Such a steel is called "Killed Steel".

Typical deoxydizing agents are aluminum, ferrosilicon (an alloy of iron and silicon) or ferromanganese (an alloy of iron and manganese). These combine with the oxygen dissolved in the molten steel to form aluminum oxide (alumina) or silicon dioxide (silica). Deoxydizing agents are added as soon as the steel is poured out from the furnace into molds and may be added individually or together, depending on the type of steel desired.

As the molten killed steel hardens in the mold, there are practically no gas bubbles seen, because most of the dissolved oxygen has been removed by the deoxydizing agents. Since there are no bubbles formed, the steel quietly solidifies in the mold and this is why it is called "killed steel". The ingot is generally free from blowholes and the distribution of carbon and other alloying elements in the steel is more uniform. This ensures that the killed steel ingot has excellent chemical and mechanical properties that are uniform throughout the entire length of the ingot. Killed steel ingots are sometimes marked with the letter "K", to indicate how they were manufactured.

Not all steel manufactured is killed, but any steel with carbon content greater than 0.25%, or in general, any steel that is meant to be forged later, is killed, Stainless steel and alloy steels are also killed as part of their manufacturing process. As we saw earlier in the series, 4140 and 4150 steels that are used in firearms have 0.40% or 0.50% carbon content. Stainless steel is also used in the firearms industry.

Sunday, November 30, 2014

In our last post, we studied one of the modern methods of steel making, the electric arc furnace. In today's post, we will study another method that is commonly used today, the Basic Oxygen Furnace (BOF) otherwise known as the Basic Oxygen Steelmaking (BOS) process.

The interesting thing about the BOS process is that the original concept is actually from the 19th century. Recall that the Bessemer process that we studied earlier, works by blowing air through hot molten metal and the oxygen in the air burns off the impurities in the molten iron. Well, the reader is probably thinking that since air consists of a mixture of nitrogen, oxygen, carbon dioxide and other gases, and since only oxygen is needed in this process, wouldn't the process become more efficient if we directly blew pure oxygen over the molten metal? The same idea occurred to Henry Bessemer (allegedly suggested to him by his father as a joke) and he received a patent on October 5th, 1858 for this concept. Unfortunately for him, this idea was not practical in the 19th century, because bottled oxygen was not available at reasonable cost or in large quantities at that time.

Also recall when we studied the Bessemer process, there are two types: the acid bessemer process and the basic bessemer process. The "basic bessmer process" is called that, because it uses an alkaline (i.e. basic) lining in the vessel (as opposed to an acidic lining). The Basic Oxygen Steelmaking process is also called "basic" because it uses an alkaline lining (usually, Magnesium Oxide (MgO)) in the vessel. The purpose of this alkaline lining is to remove elements such as phosphorus and sulfur from the molten metal, as these elements are harmful to steel's properties.

The idea of using oxygen in the furnace was revisited in the 20th century and made practical during the late 1940s. Interestingly, the modern BOS process was developed, not by any large steel companies, but mainly due to the efforts of one man and the support of a few managers in a small company that he worked for. Our story starts with a Swiss metallurgist, Robert Durrer, who graduated from Aachen university in Germany in 1915 and remained there until 1943. He served as a professor of steelmaking in Berlin's Technishe Hochschule (Berlin Institute of Technology) between 1928 and 1943, where he performed many years of experiments using oxygen for steel refining. In 1943, he returned to Switzerland and joined a small Swiss company called Von Roll AG. Here, he continued his experiments in the town of Gerlafingen, with a German colleague, Dr. Heinrich Hellbrugge. In 1947, Durrer bought a small 2.5 ton converter from the US and with it, he reported his first success in the internal plant newspaper in May 1948:

"On the first day of spring, our "oxygen man", Dr. Heinrich Hellbrugge carried out the initial tests and thereby, for the first time in Switzerland, hot metal was converted into steel by blowing with pure oxygen... On Sunday, the 3rd of April 1948 ... results showed that more than half the hot-metal weight could be added in the form of cold scrap ... which is melted through the blast produced heat"

Soon after this, two Austrian steelmakers, VOEST and Alpine Montan AG (OAMG), got interested in these developments and worked with Von Roll to commercialize this process. Theodor Suess of VOEST's plant in Linz and the managers of the Alpine Montan plant in Donawitz organized the actual experiments and worked out all the technical issues and decided to construct two 30-ton furnaces in 1949. On November 27th 1952, the first steel was produced by this new type furnace. Since the VOEST plant in Linz and the Alpine Montan plant in Donawitz were instrumental in commercializing this technology, their version is called the Linz-Donawitz process.

Since oxygen containers became available in large quantities and low cost after the 1940s, this process was very efficient and cheap. Readers interested in history might be amused to learn that the reason that methods to produce low-cost oxygen at large volumes were developed was mainly because of the German V2 rocket program! After World War II, the Germans were not allowed to manufacture oxygen in large quantities, but the factories and equipment that they had pioneered were shipped off to other countries.

In the beginning, big steel manufacturers in the US paid no attention to this innovation by a small Central European company, whose total steel making output was less than one third that of a single US Steel factory! A smaller American company, McLouth Steel in Michigan, was the first to install BOS furnaces in the US in 1954. The larger American companies, such as US Steel and Bethlehem Steel only built their first BOS furnaces in 1964. However, the rest of the world quickly adopted this new technology and by 1970, 50% of the world's steel (and 80% of Japan's steel) came from BOS furrnaces. As recently as 2011, about 70% of the world's steel output was still made using this method.

A large container, called a ladle, is lined with refractory materials, such as magnesium oxide (MgO). The ladle is tilted about 45 degrees and is charged with scrap steel and then molten pig iron from a blast furnace is also added. The ratio is about 20-30% of scrap steel to about 70-80% of molten pig iron, based on the requirements of the final steel to be produced. This takes a couple of minutes. After this, fluxes such as magnesium or lime are added to remove sulfur and phosphorus. Then the vessel is turned back to the vertical position and a water-cooled lance with a copper tip is lowered down within a few feet of the bottom of the vessel. Through this lance, pure oxygen (greater than 99% pure) is blown over the hot metal at supersonic speeds (about 2x the speed of sound). The oxygen ignites the carbon in the molten iron, forming carbon monoxide and carbon dioxide. These reactions are exothermic (i.e. they produce heat), so the temperature of the molten iron increases even more. The magnesium burns with the sulfur, forming magnesium sulfide, which is also an exothermic reaction, contributing to the rise in temperature. Silicon combines with the oxygen forming silicon dioxide slag. The blowing of the oxygen also churns the molten metal and fluxes, which helps the refining process. The slag, being lighter than the molten steel, floats on top of it.

Click on the image to enlarge

The temperature of the furnace is closely monitored and after about 15-20 minutes, a small sample of the steel is taken and analyzed to make sure that its chemistry is correct. After that, the furnace is tilted horizontally and the molten steel is tapped out into another ladle. At this point, other alloying elements such as nickel, chromium etc. may be added. Sometimes, an inert gas, such as argon may be bubbled through the ladle, to mix the alloying elements properly into the steel. To prevent slag from being poured out with the steel at the end of the tapping process, various "slag stoppers" are used, but a human eye remains the best device to determine when to stop tapping the steel. After tapping the steel out, the vessel is turned upside down and the remaining slag is poured out into a separate slag pot. The vessel is examined to make sure its refractory lining is intact and more lining material is added if needed and the vessel is prepared for the next batch.

The entire process takes about 40 minutes, which is substantially faster than the 10-12 hours that the Open Hearth Process takes. This is why it quickly replaced the open hearth process in many places around the world. Using pure oxygen instead of air makes the process more efficient and it also avoids piping nitrogen and other undesirable gases in the air through the molten steel. The process can take about 250-350 tons of metal in one charge. Unlike the electric arc furnace, this is a primary steelmaking process (i.e.) it works mostly with pig iron rather than scrap steel. This process increases the productivity of steel making -- in fact, as this process became popular, the labor requirements of steel making went down by a factor of 1000. Instead of taking 3 man-hours per ton of steel produced, it now takes 0.003 man-hours per ton of steel. The only disadvantage of this over the open-hearth process is the reduced flexibility of the charge -- the open hearth process can use up to 80% scrap steel, whereas the BOS process can only use a maximum of about 30% of scrap steel. About 70% of the world's steel today is made by the BOS process.

In our next post, we will look at some finishing up processes for steel and after that, we will look at a factory producing rifle barrels at the beginning of the 20th century.

Monday, November 24, 2014

In our last post, we studied the invention of the Siemens-Martin process to make steel. In today's post, we will study a type of furnace that was invented in the early 1900s, gained popularity around World War II and is still in use today. We are talking about the electric arc furnace.

To understand this type of furnace, we must understand what an electric arc is. An electric arc is a form of electrical discharge between two electrodes, separated by a small gap (typically, normal air). The best known example of this is lightning. Anyone who has performed arc welding is also familiar with electric arcs: you connect the work piece to the negative side of a DC power source and an electrode to the positive side, touch the electrode to the workpiece momentarily and then draw it a small distance apart from the work piece. A stable electric arc forms between the electrode and the work piece and the heat from this arc is sufficient to melt the electrode and weld the workpieces together. The same idea is used in a larger scale in an electric arc furnace.

The idea of electric arcs was first demonstrated by Sir Humphry Davy in 1810 in England and several people after him tried experiments and patented processes in the 19th century, including Carl Wilhelm Siemens, who we read about in our previous article. However, the first successful electric arc furnace was due to the Frenchman, Paul Heroult, in 1900. He was later invited to the United States in 1905, to set up furnaces for American companies, such as US Steel and Halcomb Steel. The process really gained popularity during World War II and afterwards, because of the low costs associated with setting up an electric arc furnace, compared to a complete integrated steel mill.

The furnace is a kettle made with a dished bottom, mounted so that it can be tilted forward and drained. The kettle is lined with fire brick which can withstand very high temperatures. There are doors on either side to put in raw material and the front has a spout to pour out the molten steel. The roof of the furnace is a dome lined with firebrick and has two or three carbon electrodes in it.

Electric furnaces are typically charged with scrap steel, though they may also be used with hot pig iron directly from a blast furnace. Usually though, scrap steel is used. The scrap is prepared based on the grade of steel to be made and the scrap pieces are arranged so that large heavy pieces of scrap metal don't lie in front of the burner ports. Some lime and carbon may also be added at this stage, although more may be injected at a later stage. After the charge is put in the furnace, the roof is lowered on the furnace and an intermediate amount of electricity is sent through, to start the electric arc, until the electrodes bore into the scrap sufficiently. Usually, light scrap is placed on the top of the pile to accelerate the bore-in process. After a few minutes, the electrodes melt enough of the scrap that they can be pushed deeper in and the high voltage can be fed in without fear of electric arcs hitting the roof of the furnace. As the furnace heats up, the electric arc becomes stable and starts melting the material. At this point, air (or oxygen) may be fed into the furnace to burn up the carbon, silicon, manganese etc. and form steel. More carbon and limestone and other elements may be added at this stage to form the steel.

As we have studied before, phosphorus and sulfur tend to weaken the steel and must be removed. As it turns out, the conditions favorable to remove phosphorus are opposite to those favorable to remove sulfur and vice versa. As a result of this, there is a chance that one of these elements may revert back into the steel from the slag, if proper steps are not taken. Therefore, the phosphorus removal is carried out very early on in the process -- while the temperature is still relatively low, the furnace is tilted to pour out the initial slag formed, which gets rid of much of the phosphorus. If this high phosphorus slag is not removed early on, it will revert back into the steel later on. Then the furnace continues to be heated, and more slag formers are introducted to remove the other elements, such as silicon, sulfur, calcium etc.

The molten metal is analyzed via a spectrometer to make sure that the carbon content and oxygen are correct. Once the correct temperature and chemical contents are achieved, the steel is tapped out as shown in the illustration above. At this point, beneficial alloying elements such as nickel or vanadium may be added to the tapped metal stream.

After all the metal is tapped out, the solid slag is cleaned out of the vessel, the electrodes are checked for damage and the new charge is prepared to be introduced into the vessel. The entire process of preparing a charge, melting it, tapping it, cleaning out the vessel and recharging it, takes about 60 minutes on a medium-sized furnace (capacity of 90 tonnes or so).

Electric arc furnaces can range from really small sizes suitable for research labs to large ones capable of working with 400 tons of metal at a time. The nice thing about them is that they can work with 100% scrap metal, which means they are very handy for recycling old steel, which can be bought for far cheaper than iron ore. They can easily be started and stopped, unlike other furnaces. They are also very energy efficient, compared to methods that make steel from raw iron ore. They can produce very high grade steel from cheap and impure metals and even better than the Siemens-Martin process. Since they run at higher temperatures, they allow the operator to make slags that are normally difficult to melt, but useful to remove small traces of impurities. They can be used for superior stainless steel alloys as well. Nucor, one of the largest steel manufacturers in the US, uses electric arc furnaces a lot, because it allows them to put up smaller mini-mill plants near where the steel is needed and they can vary production quickly, depending on the demand.

Electric arc furnaces are also used as part of the process in vaccuum arc remelting (VAR), which is used to produce specialty steels. In this process, the steel is first melted in an electric-arc furnace and then alloyed in an argon oxygen decarburizing vessel and poured into ingots. Then, the ingots are put into another container and the air is removed from it to form a vacuum. An electric arc is used to remelt the steel, since the arc can form without the need for oxygen. Any dissolved gases (such as nitrogen and oxygen) escape out under the vacuum conditions, as do elements such as sulfur and magnesium, which have high vapor pressure. The molten steel is solidified at a controlled rate, using a water jacket around the vessel to control the cooling rate and ensure uniformity. It is known that the VAR process is used to produce 9310, 4340, Aermet 100 and maraging steels, which we studied earlier when studying steels used for rifle barrels, bolts and firing pins at the start of this series. The process can also be used to produce titanium, which is also sometimes used in the firearms industry, as we studied before.

Wednesday, November 19, 2014

Two posts ago, we studied details of the Bessemer process, which revolutionized the production of steel and dropped the price of steel to be comparable to that of iron. Today, we will study another process that was developed to complement the Bessemer process. This process is called the Open Hearth Process, or the Siemens-Martin Process.

As we noted two posts ago, the Bessemer process is a very fast process and converts iron to steel in around 20 minutes or so. One problem with this is that it is very hard to control the carbon content precisely, because it is not possible to sample the molten metal at an intermediate stage. Also, the output is less homogenous and could have blowholes and cracks in it (and we saw one way to fix this problem in our last post on fluid compressed steel). The Open Hearth process fixes some of these issues.

The origins of the Open Hearth process go back to the work of Carl Wilhelm Siemens, a German engineer, who moved to England and changed his name to a more British sounding name: Charles William Siemens (and later, he was knighted and called Sir William Siemens). Carl Siemens was the younger brother of Ernst Werner Siemens, the famous German inventor of numerous electrical technologies and later, a co-founder of Siemens AG, the German telecommunications and electrical company. They came from a large family of 14 children and when both parents died, the older brothers took responsibility to support their younger siblings education. Ernst Siemens had shown early interest in electricity and worked on improving existing technologies. One of his early inventions was a better method of electroplating gold and silver onto metal items. Meanwhile Carl Siemens had just finished graduating as a mechanical engineer and the brothers were wondering how to support their younger siblings, so they decided to earn money by licensing Ernst Siemens invention to a British company. Since Carl Siemens had studied English in school and spoke it better than his brother, it was decided to send him to England to act as his brother's agent and market his patent there. Carl Siemens moved to England in March 1843 and liked it so much that he made England his new home. Since he was also an engineer at heart, he liked to devote his spare time to various researches.

One of Carl Siemens early subjects of research was how to improve the efficiency of furnaces and he came up with the concept of a regenerative furnace in 1857. The idea is to use some of the heat from the exhaust gases to preheat fresh air coming into the furnace. This allows the furnace to use less fuel overall. The initial furnaces that Siemens built were used for glass-making and his idea saved about 70-80% of the fuel that was previously used. By the early 1860s, he had built a small factory to produce wrought iron from iron ore and pig iron using his regenerative furnace. In 1865, a French engineer named Pierre-Emile Martin, licensed the Siemens regenerative furnace patent and modified it to be used to make steel and he started a small factory in France. After this, Siemens used Martin's modifications and set up a small steel manufacturing plant in Birmingham in 1866 and later, a larger factory in Swansea in 1869,that produced about 75 tons of steel a week. By 1870, the open-hearth process was perfected and called the Siemens-Martin process after its inventors.

To understand the process, we must first understand the principle of a regenerative furnace. The furnace has at least two chambers, one on either side of the main hearth. The furnace has dampers to regulate the direction of the flow of air and flammable gas. The air and gas are allowed to flow in one direction through one of the chambers, then they are mixed together, ignited and allowed to pass over the hearth, thereby transferring some of the heat to the iron ore to be melted. As the hot gases leave the hearth and move towards the chimney, they are transferred to another chamber lined with fire bricks, where some of the heat of the exhaust gases are transferrred to the bricks. After about 20 minutes, the flow of air is reversed by turning the dampers, therefore the air comes in through the hot chamber and is preheated before it is mixed with the flammable gas. This allows more heat to be produced in the hearth and the hot exhaust gases are piped through the first chamber, heating the bricks in it as well. Every twenty minutes or so, the direction of the air and flammable gas are reversed. This method allows the temperature of the furnace to be hot enough to melt steel.

A regenerative furnace. Click on the image to enlarge. Public domain image.

In the above image, we see a regenerative furnace. The gas enters from the center of the image and the air enters from the bottom. In the above figure, we see the dampers are initially set to allow the air and gas to enter the chambers on the left side and then combine together and ignite over the hearth. The hot exhaust gases are then led over to the two chambers on the right, where they heat up the bricks in the chamber. The hot exhaust gases are then discharged via the chimney. After twenty minutes or so, the dampers are moved so that the flow of air and gas are reversed. Now, the air and gas flow in through the chambers on the right side and flow out of the chambers on the left. Since the bricks in the chambers on the right side were heated earlier, they now transfer their heat to the incoming air and gas. The burnt exhaust gases are led through the two left chambers to heat up the bricks in there. The flow of air and gas is reversed every twenty minutes and the process continues until the contents of the hearth are melted fully.

The contents of the hearth may be loaded with scrap steel, sheet metal, pig iron, construction steel, iron oxide (rust) etc. Once the steel is melted, slag forming agents such as limestone can be added to remove the impurities. The slag floats on top and can be removed when the furnace is tapped. The oxygen in the air burns off the excess carbon in the steel. If more carbon or other elements are needed, they can be added after the molten steel is tapped from the furnace. In the early days, this was not an easy process. According to one US worker from 1919, he described this process as follows: "You lift a large sack of coal to your shoulders, run towards the white hot steel in a hundred-ton ladle, must get close enough without burning your face off to hurl the sack, using every ounce of strength, into the ladle and run, as flames leap to roof and the heat blasts everything to the roof. Then you rush out to the ladle and madly shovel manganese into it, as hot a job as can be imagined!"

Unlike the Bessemer process that can only work with pig iron, this process can use scrap iron, scrap steel and waste metal, as well as pig iron, in the hearth. The Siemens-Martin process is much slower than the Bessemer process and takes about 8-10 hours to complete, but it has some advantages as well. For one, a small sample of the molten metal can be taken out of the furnace and allowed to cool and then taken to a lab for testing, to make sure the carbon content is perfect. The long heating process makes the content of the steel more homogenous than steel produced by the Bessemer process. It is not necessary to remove all the carbon at first, like the Bessemer process, as the longer time of the Siemens process allows the operators to precisely control the carbon content and they can stop it when the required amount of carbon has burned off. This process also allows recycling of scrap steel. One more nice thing is that steel from different sources (with different amounts of carbon content) can all be combined into a single furnace and converted to a new steel with the given amount of other elements in it. Since scrap steel and old sheet metal are often obtained for cheap, the cost savings is considerable. Also, this process allows people to recycle old worn out steel, such as old rails, construction steel beams, scrap metal from junkyard car bodies etc., instead of paying to dispose of the junk. In the Bessemer basic process, the phosphorus remains in the liquid metal until the carbon is all burned off, but in the open hearth process, much of the phosphorus is removed earlier on in the process. When steel of a uniform character is desired, the open hearth process is preferred, but if large amounts of steel are required quickly, then the Bessemer process is used.

The Bessemer process and the open hearth process complemented each other and were used in many places in the world up to the late 1980s or so. In America, the last open hearth furnace was shut down in 1992, but some open hearth furnaces are reportedly still in use in India and Russia. In many places, the open hearth furnaces were replaced by oxygen lances and electric arc furnaces, which we will study in the next few posts,

Monday, November 17, 2014

In our last post, we studied how the Bessemer process made it possible for the first time for steel to become as cheap or cheaper than cast iron. The quality of steel wasn't as high as the crucible process, but the price of steel was now much more affordable, which meant there were more manufacturers offering steel barrels in their firearms.

One of the problems with manufacturing steel with the Bessemer process was that passing air through the molten metal sometimes formed gas bubbles and these would form internal blow holes in the steel ingot as it cooled down. Also, a steel ingot shrinks as it cools and this shrinkage could also cause gaps and hairline cracks to form in the ingot. These flaws in the ingot could weaken the final product, if not properly eliminated.

The solution to this problem was found by the famous British engineer, Sir Joseph Whitworth. We had studied about his inventions when we studied the Whitworth rifle many months ago. This innovative rifle was much more accurate than its competition, but was more expensive to manufacture, which is why it was rejected by the British military. However, it was used by other people who had need for accurate rifles, such as confederate snipers, during the US Civil War. Sir Whitworth continued to make improvements to his rifle and discovered a way to remove most of the flaws in steel ingots. His method of manufacturing steel was called fluid compressed steel, and we will study the process in today's post.

We know that Whitworth first discovered his method in 1865, because he registered a patent during that year, but it wasn't until 1869 that he finished building the machinery to manufacture the steel in quantity.

The steel is manufactured using the process we studied in our last post, but when the steel is poured out into a mold, instead of allowing it to cool by itself, a hydraulic press is used to apply pressure to the ingot while it is still in a liquid or semi solid state. This pressure causes most of the generated gas bubbles and cracks in the ingot to collapse or move towards the ends of the ingot. To give some idea of the pressures involved, an ingot measuring about 15 feet (4.57 meters) in length before compression,decreases about 12 inches (30.5 cm) in length after compression, to form a bar of length 14 feet (4.27 meters). After the steel solidifies, the two ends of the ingot are cut off (about 20% of the length) and discarded and the remaining bar contains far less bubbles and cracks. This bar can now be used for various applications, such as making quality steel barrels.

A Whitworth Hydraulic Press at the Armstrong-Whitworth company. Notice the man standing on the left side of the press.

Click on the image to enlarge. Public domain image

Whitworth fluid compressed steel was generally acknowledged by many gunmakers, to be of excellent quality and Whitworth's name became a selling point. Therefore, many firearm manufacturers of that era would often stamp Whitworth's name and trademark (a sheaf of wheat) on their barrels, alongside their own names, to show that these barrels were made of superior quality steel.

Another double-barreled gun showing the Whitworth trademark (a sheaf of wheat). Click on the image to enlarge.

Whitworth Fluid Compressed Steel was used by many high end manufacturers, such as Purdey and W.W Greener in England, and Parker, Remington and LC Smith in the United States.

Whitworth's patent for fluid compressed steel expired in 1879, but a special committee of the British government extended his patent for 5 more years. After the patents finally expired in 1884, many other manufacturers started making their own versions of fluid compressed steel, The most famous competitor of Whitworth was Krupp Steel works from Essen, Germany, who made their own Krupp fluid compressed steel. Like Whitworth, many manufacturers began to advertise that they used Krupp steel in their barrels.

A pair of barrels made of Krupp Fluid Steel. Click on the image to enlarge.

Some famous American manufacturers like Lefever, Stevens and Ithaca were known to use Krupp's steel, as well as German manufacturers, such as JP Sauer & Son.

Krupp and Whitworth were the two famous manufacturers of fluid compressed steel, but there were also other manufacturers such as Jessop, Sterlingworth, Chromox etc. Fluid compressed steel continued to be used in barrels till about 1925 or so, while other ways of manufacturing steel to eliminate gas bubbles were discovered. We will study some of these other methods in a couple of posts.

In the next post, we will study the open hearth process to manufacture steel and then move on to more modern methods.

Saturday, November 8, 2014

In our last post, we saw how crucible steel was manufactured after around 1740 or so, using the process invented by Benjamin Huntsman. While crucible steel was a significant improvement over blister steel in terms of quality, it was still somewhat expensive to produce. Therefore, many firearm manufacturers used steel for smaller parts, such as sear springs, frizzens etc. and many barrels were still made of wrought iron, instead of steel. As we saw in our previous post, some larger manufacturers like Remington and Colt did offer superior steel barrels after 1820 or so, but they cost over double the price of wrought iron barrels and therefore, both companies sold wrought iron barrels as well, as a cheaper alternative to their steel barrels. High end firearm manufacturers combined steel and iron to make damascus barrels. These were beautiful to look at, but they were expensive to produce and generally designed for rich clients.

So what was the reason for the higher cost of steel. Well, let's look at the processes involved to convert iron ore to steel using the crucible steel method, as done before the 1850s:

All four steps needed to be done to produce crucible steel, whereas producing wrought iron only required the first two steps. Steps 2, 3 and 4 also required skilled workers with specialized training (we studied about specialized workers called puddlers, puller-outs and teemers in the last few posts). Step 2 was also not geared towards mass production. Using finery forges was a slow process and work-intensive in nature. While the puddling forge replaced the finery forge, it also required specialist workers and puddler workers generally had short life spans as well, due to the unhealthy and stressful nature of their work. Step 3 took wasn't a continuous process either and took the longest time to finish (typically, a batch would take 2 weeks to convert from wrought iron to blister steel). Step 4 was also done in batches, since it was limited by how much puller-outs and teemers could lift at a given time. Step 4 also typically took around 4 hours to finish. No wonder, crucible steel/cast steel cost so much more than wrought iron.

Improvements in the crucible steel manufacturing process, done in the United States in the middle of the 19th century, rendered step 3 unnecessary, as it was now possible to convert wrought iron to crucible steel directly in the crucible. However, the improved process still took a few hours to accomplish, was still a batch process and required skilled workers. Therefore, wrought iron was still the material of choice for many gun makers. Incidentally, large construction projects like bridges and towers of this era also generally used wrought iron, because of the non-availability of large volumes of steel to meet the demand.

The price of steel did not drop until an English engineer named Henry Bessemer invented the Bessemer process in 1856. With his invention, the cast iron produced in step 1 above could be directly converted to quality steel, without going through steps 2, 3 and 4. It could also be produced in larger volumes than using the crucible process and could be done in 30 minutes, further reducing costs. In fact, the steel produced by his method cost the same price or cheaper than wrought iron. Since steel is generally harder and tougher than wrought iron, after this low-cost production method was invented, most industries stopped using wrought iron altogether and switched to steel completely. In fact, in today's modern world, the only people producing wrought iron are traditional blacksmiths in tiny shops employing only one or two people. We will study how the Bessemer process worked in today's post.

The process consists of melting cast iron in a large vessel (called a Bessemer converter) and blowing air through the molten iron from the bottom of the vessel, through nozzles called "tuyers". The oxygen in the air oxidizes impurities such as silicon, manganese and excess carbon and forms oxides, which either escape as gases or form lighter slag which floats on top of the molten iron and can be separated. The oxidation of impurities also raises the temperature and keeps the iron in a molten state. The materials used to line the insides of the Bessemer converter vessel also play an important part in removing some impurities, as we will see below. The production of oxides causes a large flame to appear in the mouth of the vessel and monitoring this flame gives an indication of how the oxidation process is proceeding. After the oxidation is complete, the slag is removed and a precise quantity of carbon and other elements are mixed into the molten metal to form steel. This molten steel is then poured into molds to solidify.

A Bessemer converter. Click on the image to enlarge. Public domain image.

The process of converting cast iron to steel only takes about 20 to 30 minutes and doesn't use as much coke as some of the other processes we've studied in the past. Also, large vessels can be built to handle about 30 tons of metal at a time, making it more efficient for producing large volumes of steel. Typically, a factory has at least two converter vessels for efficiency, so that while one vessel is being filled or emptied, the other one is busy melting the iron.

The process of oxidizing iron (decarburizing) with forced air was actually known to people outside Europe, many centuries before the Bessemer process was invented. We know that the Chinese had a decarburizing process in the 11th century AD and there are European traveler accounts of Japanese using a similar process in the 17th century. However, they produced steel in smaller quantities only. It was Henry Bessemer, who converted this process into a large scale industrial production process and we therefore know it as the Bessemer process.

The invention of the Bessemer process was due to a lucky accident. The Crimean war had started and Henry Bessemer happened to meet King Napolean III in 1854 in Vicennes, France and had a short conversation with him, where the King said that what the world needed was for someone to invent a better and cheaper way to produce steel in quantity, so it could be used for guns (both firearms and cannon were largely made of wrought iron at this time). Henry Bessemer started working on the problem in 1855 and patented the process in 1856. A lucky discovery by him actually gave him an insight into the process. He was working with a puddling furnace and by chance, some of the wrought iron pieces ended up on the side of the puddling chamber and were exposed to the furnace's heat for a while. When he went to push those pieces back to the middle, he discovered that the pieces had been converted to steel. This gave him the idea to rework the furnace to push high pressure air via pumps through the iron. "But wait a minute", the reader asks. "Won't blowing air on top of an object cool it down? People blow air via their mouths to cool down hot coffee or hot soup, so why doesn't blowing air cool down the iron?" Well, hot coffee or hot soup don't contain impurities that burn, whereas cast iron does. The oxygen in the air causes the impurities to burn, which increases the temperature of the vessel, which in turn burns more impurities and increases the temperature of the vessel even more, until the iron melts completely. The first impurities to burn are the silicon and carbon in the pig iron, followed by the rest of the impurities.

In order to make the process more popular, Bessemer licensed his process out to four different vendors in different geographic areas, with the plan of gaining market share for his method. He sold the process to the four vendors for a total of £27,000, but none of them could make it work successfully and he ended up getting sued in court! In the end, he bought back his patent licenses for £32,000 and built his own factory. In his initial process, his method consisted of burning off just enough impurities to reduce the carbon content to the required amount to make the grade of steel desired and then stopping the flow of air. Well, that was the theory anyway, but it didn't work so well in practice and he spent large sums of money unsuccessfully trying to figure out how to determine when to stop blowing the air. Another issue was that certain impurities in the steel also react with nitrogen gas, which happens to be a large part of air as well.

It was left to another British metallurgist, Robert Mushet, to provide the solution. Before the Bessemer process was invented, Robert Mushet had discovered in 1848, that adding a small amount of spiegeleisen (an alloy that is rich in carbonates of iron and manganese mainly, with a little carbon and silicon as well) to steel made it much easier to work with when heated. The sample of spiegeleisen was brought back to him by a friend who had returned from a tour of the Rhineland area in Germany and thought that he might like to look at the shiny mineral (spiegeleisen is very shiny and the name literally means "mirror iron" in German). We now know that adding manganese to steel has the effect of increasing the malleability of steel, as we saw earlier when we first started this series.

A sample of Spiegeleisen. Click on the image to enlarge. Public domain image.

Shortly after the Bessemer process was invented, another friend, Thomas Brown, knowing of Robert Mushet's interest in metallurgical problems, brought him a sample of poor quality Bessemer steel and challenged him to improve it. His solution was very simple and was overlooked by everyone else, including Henry Bessemer. Instead of trying to determine when the level of carbon content in the steel had reached the required level and then stopping the flow of air, he instead kept pumping in more air until the entire content of carbon and other impurities had burned off. After all the carbon and impurities had been burned off, the flames would no longer shoot out of the front of the furnace thus indicating that they were all burned off, that's when he stopped the flow of air and added a precise amount of spiegeleisen back into the molten iron, to add back the required amount of carbon and manganese and form high quality steel. This improvement made it much easier to produce steel rails and bars. He also invented other processes to improve the casting of steel (his method is still used today) and also developed the first true modern tool steel. Robert Mushet dreamed that he and Bessemer would become rich men by his inventions, but he didn't manage to profit by them at all, whereas other people did. By 1866, he was bankrupt and ill and his 16 year old daughter went to London alone and angrily confronted Henry Bessemer in his private office and told him that he wouldn't have become rich without her father's invention. Henry Bessemer saw the logic in her argument and paid Mushet a pension of £300 annually (which was a big sum of money in those days) until he died in 1891.

There was also another problem with Henry Bessemer's process. Well, it really wasn't a problem for him, because he was in England and English iron was low in phosphorus content. Remember the section above, where we mentioned that the lining of the Bessemer converter vessel also plays a role in removing some impurities from cast iron. Bessemer lined his vessel with clay and it worked very well with cast iron with low phosphorus content. The process using a clay lining is called acid Bessemer. The trouble is that in the rest of Europe, their cast iron contained a larger amount of phosphorus and this impurity wasn't removed by the clay lining, which resulted in low-grade steel being produced (phosphorus weakens steel). A British chemist by the name of Sidney Gilchrist Thomas solved this problem in 1876, with the help of his cousin, Percy Gilchrist. His solution to the problem was to coat the inside of the vessel with a lining of dolomite or limestone, which removes the phosphorus impurities. This process is called the basic Bessemer process, as the lining is alkaline in nature (as opposed to the acid nature of the clay lining). It is also called the Gilchrist-Thomas process, after its inventor. The process actually generates more slag than the acid Bessemer process. As an extra bonus, the high phosphorus content of the slag meant that it could be sold to farmers as a fertilizer, thereby increasing the profit of the factory! The invention of the basic Bessemer process was very valuable to European countries like Germany and Belgium, where the iron had high phosphorus content and Thomas' name became much more well-known in those countries than in his native England! In the United States, even though more iron ore is low in phosphorus, his method still found lots of supporters here too.

The Bessemer process quickly made Sheffield a major producer of steel. In America, a team of investors went over to England in 1863, to license the technology, with a view to using it to improve shipbuilding, armor and armaments. They built their first factory in Troy, New York, in 1865, to manufacture steel rails for trains. The main American engineer involved, Alexander Holley, continued to improve the Bessemer process and built or consulted for about a dozen different steel plants between 1866 and 1877, including the first Pennsylvania Steel plant for the Pennsylvania railroad company. An early investor who saw great potential in the improvements made by Holley was Andrew Carnegie. who hired Holley to build the Edgar Thomson Steel Works in 1873, located in Pittsburgh. This was one of the largest steel plants in the country at that time and helped make the United States a world leader in steel production, overtaking Britain by 1890 or so. Manufacturing steel made Andrew Carnegie one of the richest men in America and towards the end of his life, he donated his vast fortune to various causes, including funding thousands of public libraries and some universities (he's well known for his contributions to Carnegie Mellon University, but what is not as well known is that he also donated large sums of money to the Tuskegee Institute in Alabama and the University of Birmingham in England). The Edgar Thomson plant is still in service, now part of US Steel, and this factory currently produces about 28% of US Steel's production in America. About 900 people work in here, many of whom had fathers, grandfathers and great-grandfathers working in the same factory as well.

With the invention of the Bessemer process, not only did the time taken to produce steel from pig iron drop significantly (it was faster to produce than even wrought iron!), it was more efficient and could work with larger volumes of cast iron as well. The cost of producing good-quality steel dropped from about £60 per ton to about £7 per ton, shortly after Bessemer started his first factory. With improvements to the process made by others, the prices dropped even more. For instance, an invention by William Jones, while working in the Edgar Thomson steel plant, improved the Bessemer process to become a continuous process. flowing molten iron directly from the blast furnace to the bessemer converter. As a result of this, steel began to replace wrought iron in many applications, as it was now cheaper to produce, as well as being tougher and stronger than wrought iron. The Bessemer process started declining in England around 1895, but it continued in other places in the world for a lot longer. Germany produced most of its steel in the 1950s and 1960s using this process, and in America, the last factory using the Bessemer process closed in 1968. One of its issues was actually its speed of production -- it ran too fast! Given that the steel could be produced in under 20 minutes, this gave little time to analyze the steel and make sure that it has the alloying elements in the correct proportions and to adjust the percentages as needed. The flame produced by burning the impurities is large and spectacular and while it is burning, people cannot approach the vessel to take samples, therefore the amounts of various elements in the steel cannot be adjusted midway through the process. One of the later improved Bessemer processes (the oxygen lance process) replaced the Bessemer process in many places. The oxygen lance process blows pure oxygen instead of air, over the molten metal, to better improve oxidation. Interestingly, the oxygen lance method was actually patented by Henry Bessemer in the 19th century, but he could never build it with the available 19th century technology, because of the difficulty of obtaining large quantities of oxygen.

We will study some more improvements in steel making in the next few posts.

Sunday, November 2, 2014

In our last post, we saw how wrought iron could be converted into steel, by adding carbon to wrought iron in a closed furnace, in a controlled manner. Recall that, in our previous post, we mentioned that the problem with this method was that the distribution of carbon throughout the steel bar was non-uniform, resulting in some parts of the bar being harder than other parts. As we saw in the previous post, one way to handle this was to shear the blister steel bars into smaller pieces, stack the pieces on to a pile, re-heat the pile and then weld them together, so that the carbon content would be more evenly distributed. For better product, the process would be repeated multiple times. However, all this increased the cost of the steel and it did not necessarily result in even distribution of carbon in the steel either.

By the early part of the 1700s, steel was being used to make some parts of firearms (e.g.) lock springs, frizzens etc., as well as the tools to make firearms. In Europe, England and Germany were two major sources of steel during this period. The next development in steel making was due to an English clock maker and the technology he developed was crucible steel. We will study the process in this post.

The process of making crucible steel is actually much older -- as early as 300 BC, there were several places in southern India making a type of crucible steel called "wootz steel". This steel was exported to the middle-east, where it was encountered by Europeans during the crusades and was labelled by them as "damascus steel". The source of iron ore for the Indian steel was an area in South India, where the iron ore came with small amounts of vanadium and other rare earths. As a result of these trace elements, wootz steel has carbon nanotubes in it, contributing to its superior ability to hold an edge. Unfortunately, by the 1700s, with the rise of British power in India, the secrets of its production died with the blacksmiths. However, we have several earlier descriptions of many travelers to India (Arabs, Persians, French, English, Scottish etc.) from which we know that they were definitely using a crucible process.

Over in England, Benjamin Huntsman was in the business of making clocks, tools and locks in Doncaster, in the early 1720s. Later on, he also practiced as a surgeon and an oculist. Like most people in the clock making trade, he bought most of his steel from German sources. However, he found that this steel was not always good enough for springs and pendulums for his clocks, where consistency in the steel is the key to accuracy. Therefore, he performed several experiments to try and find a more uniform steel production process. Since he needed a large amount of suitable fuel for his steel furnace, he moved his business from Doncaster to Sheffield in 1740, because of the better availability of coke and coal in Sheffield. He continued his experiments in secret in Sheffield for many years and gradually re-discovered the crucible steel process. Essentially, his process consists of melting the iron in a clay crucible, adding a precise amount of carbon. The carbon distributes evenly throughout the molten steel, resulting in a more consistent product. The molten steel is then poured out into a mold to harden. Since the steel is poured out into a mold, it is sometimes called "cast steel" as well. However, unlike cast iron, this steel is flexible enough that it can be heated and forged by a hammer as well, or even welded.

The process starts off by using a crucible made of clay, to which is added wrought iron bars and powdered charcoal. The amount of charcoal added to the crucible is calculated based on the amount of wrought iron. A flux consisting of ordinary glass pieces is also added to the crucible. The crucible lid is then sealed and it is heated in a furnace. Since the glass has a lower melting point than the iron, it melts first and forms a liquid in the bottom of the crucible. After a few hours, the iron starts to melt and absorbs some of the carbon from the powdered charcoal as it becomes a liquid. Since iron is denser than glass, the liquid iron sinks past the liquid glass to the bottom of the crucible. Any oxygen is released in the form of carbon monoxide gas, which bubbles out through the layer of liquid glass. In a few hours, the iron is fully melted into a liquid and absorbs enough carbon to transform to steel. The liquid steel is at the bottom of the crucible, with a layer of liquid glass above it. The liquid glass seals the steel and prevents any oxygen or excess charcoal carbon from being absorbed by the molten steel. At this point, a worker, called a "puller-out", (sometimes, it was two people) reaches down into the furnace and pulls out the crucible pot. The crucible pot can be left to cool until the metal turns solid, at which point, the glass layer is broken with a hammer and the steel ingot underneath is retrieved. Alternatively, immediately after pulling the crucible from the furnace, another worker, called a "teemer", can open the crucible lid and pour the liquid steel into a mold, with another worker using a tool to dam the glass slag floating in the crucible on top of the steel. The steel has to be poured into the mold quickly (in under two minutes or so) and then a lid is placed on the top of the mold, to limit the amount of oxygen combining with the cooling steel. In about five minutes, the steel becomes solid enough inside the mold. If the steel ingot is to be sold to someone else, then the mold is allowed to cool for several hours before being opened. However, if the foundry has its own forging shop, then the mold is broken after 5 minutes and the still hot ingot is carried off to a hammer to be forged into the final shape, as it is still soft enough to be easily shaped (incidentally, this is the origin of the English saying, "strike while the iron is hot"). The crucible can be re-used a few times before it has to be disposed off, because it weakens due to the intense heat and erosion, every time it is used.

The "puller-out" and "teemer" had to be strong men, to lift and handle the crucible, since the weight of the steel alone in a single crucible was usually around 20 to 45 kg. (45 to 100 lbs.). The mold was typically about 50-100 cm. (about 20 to 40 inches) in length and square in cross section. It was made of two halves, held together by rings. The hole on top of the mold typically had a width of only 7.5 cm. (about 3 inches). The mold was deliberately kept narrow so that the molten steel cannot be exposed to much oxygen as it is poured into the mold. A good teemer could pour molten metal from the crucible through this narrow hole of the mold in under 2 minutes, without any splashing or spilling. Teemers were trained to do this by making them pour cold lead pellets into molds, until they could do it perfectly, before they were allowed to handle hot steel.

A teemer at work. Public domain image.

In the beginning, Huntsman remelted a mixture of blister steel and wrought iron, instead of just wrought iron, in his crucibles, and he kept improving the process over several years. He realized very early on, that his steel could be used for other purposes besides clock springs and tried to interest other local manufacturers of cutlery and tools to use his steels, but they were not interested, since his steel was harder than everyone else's steel. Therefore, he exported his steel to France, where it was very well received. Pretty soon, the Sheffield cutlery manufacturers began to lose market share to superior products from French manufacturers and as a result, they actually tried to obtain a government order to force Huntsman to stop exporting his steel! Due to their efforts, Huntsman even contemplated moving his factory elsewhere. Luckily, cooler heads prevailed and the Sheffield manufacturers abandoned their attempts to sabotage his business and started buying from him instead and the demand for his steel went up tremendously. He established a larger steel factory in 1770 and the city of Sheffield started becoming famous for its steel. Within 100 years of his discovery, the city of Sheffield was producing about 40% of the steel produced in Europe!

Click on the image to enlarge.

Huntsman worked in secret and never patented his process, so other companies elsewhere also tried manufacturing crucible steel. However, they could not duplicate the Huntsman process immediately for a few reasons.

The first reason was the crucible -- it had to be able to withstand high temperatures and therefore, it needed to be made of a special type of fire-clay. By some lucky chance, the place where Huntsman went to dig his clay from, in the north western part of Sheffield town, happened to be one of the few places in England where this special type of clay existed. We now call this type of clay as "Stannington clay". When people in other parts of England, Europe and the United States tried to duplicate the process, their attempts failed because their clay pots could not withstand the intense heat of molten steel. It took other people a few decades to figure out that the type of clay used was crucial to the process.

The second reason was the flux that he used -- his secret was broken glass. The glass melts before the steel does and coats the surface of the molten steel ingot. As legend has it, this secret was finally discovered by one of his competitors, using industrial espionage tactics. The story goes that a person by the name of Samuel Walker had a rival foundry at Grenoside, on the northern part of Sheffield. One cold winter night, Walker disguised himself as a poor beggar and showed up outside Huntsman's factory, pretending to be ill and begged to be let inside for shelter and warmth. The workers took pity on him and led him to a corner of the factory floor to sleep in. Walker pretended to sleep, but what he was actually doing was carefully watching the whole process of making the steel. He observed the workers breaking green glass bottles and putting them in the crucibles. About three months later, Walker's factory in Grenoside was also making crucible steel. Whether the story about the disguised beggar is true or not, it is definitely true that Samuel Walker did exist and he did learn details of Hunstman's secret process somehow. Samuel Walker is recorded to have built his rival factory for making steel in 1750, although he did not expand his factory until 1771, indicating that his original furnace had only limited success. Perhaps he didn't figure out the other secrets, such as the clay, until many years later. Other people in Sheffield also started making cast steel, once they had figured out Huntsman's secrets and Sheffield became the first "Steel city" in the world.

The following three videos show some experiments made by a couple of geeks (one is Niels Provos, who is well known in computer security circles and now works in Google):

In the United States, steel was mainly imported from England during this period. The Remington company was one of the first to start offering crucible steel barrels for firearms in the late 1820s. In 1845, Samuel Remington appeared before the Ordnance Trial Board, to persuade them to use Remington steel barrels for military firearms.

By the time of the Civil War, both Remington and Colt were supplying crucible steel barrels, while most of the other manufacturers were still making wrought iron barrels only. Both companies stamped "cast steel" on their barrels, to show that they were of a superior quality. It must be noted though that wrought iron barrels were still cheaper than cast steel at this stage, so both companies also offered wrought iron barrels for sale as well. From a catalog dating from 1871, Remington is listed as offering both cast steel barrels and iron barrels of different grades. During this time, Remington's "cast steel barrels weighing 6 lbs. or less" are listed at a price of $5.00 each, whereas the price of their "iron barrels weighing 7 lbs. or less" are listed at $3.00 each.

The invention of the Bessemer steel process dropped the price of steel even more and was really responsible for many other firearm manufacturers to switch from wrought iron to steel. We will study that process in the next post.